The present invention relates to microelectromechanical system (MEMS) actuators and, in particular, to thermal microelectromechanical system actuators that are activated by Joule heating.
Microelectromechanical system (MEMS) actuators provide.control of very small components that are formed on semiconductor substrates by conventional semiconductor (e.g., CMOS) fabrication processes. MEMS systems and actuators are sometimes referred to as micromachined systems-on-a-chip.
One of the conventional MEMS actuators is the electrostatic actuator or comb drive. Commonly, such actuators include two comb structures that each have multiple comb fingers aligned in a plane parallel to a substrate. The fingers of the two comb structures are interdigitated with each other. Potential differences applied to the comb structures establish electrostatic interaction between them, thereby moving the comb structures toward and away from each other.
Advantages of the electrostatic actuator are that they require low current, which results in small actuation energy, and have a relatively high frequency response. Disadvantages are that they require high drive voltages (e.g., tens or hundreds of volts) and large areas and provide low output forces. For example, this type of actuator can produce a force of 0.0059 nN/volt2 per comb-finger height (xcexcm) and can yield an actuator force density of about 20 xcexcN/mm2, with the area referring to the surface area of the actuator. Comb drive (electrostatic) actuators used for deployment of microstructures typically occupy many times the area of the device they are deploying. Also, the high voltages (e.g., tens or hundreds of volts) required to operate electrostatic actuators can be incompatible and prevent integration With conventional logic and low voltage electronics.
A pseudo-bimorph thermal actuator is an alternative to the electrostatic actuator. These actuators utilize differential thermal expansion of two different-sized polysilicon arms to produce a pseudo-bimorph that deflects in an arc parallel to the substrate. Such a thermal actuator produces much higher forces.(100-400 times) than comb drive actuators and can operate on very low voltages and can achieve about 450 xcexcN per/mm2 of MEMS chip area. A disadvantage is the additional electrical power that is required and the sweeping or arc motion of the actuator. Two or more actuators may be coupled to a common beam through bending yokes to produce a linear movement, which is usually desired in MEMS systems. However, the bending of such yokes consumes much of the force output of the actuators.
The present invention includes an in-plane thermal buckle-beam microelectrical mechanical actuator formed on a planar substrate of semiconductor material, for example. The actuator includes first and second anchors secured to the substrate and a floating center beam positioned between the first and second anchors and movable relative to the substrate. Symmetric first and second sets of elongated thermal half-beams are secured between opposite sides of the floating center beam and the respective first and second anchors. The half-beams are formed of semiconductor material, such as polysilicon. A current source directs electrical current through the thermal half beams via the anchors to impart thermal expansion of the thermal half-beams and hence linear motion of the floating center beam generally parallel to the substrate.
In one implementation, the half-beams are configured at a bias angle to give the floating beam an affinity for in-plane motion. An actuator of the present invention with such bias angles can give the actuator an overall chevron configuration.
Actuators according to the present invention provide linear output motions, in contrast to conventional thermal actuators that rotate about an axis and must have mechanical linkages to convert the rotational motion to linear in many cases. The resistivity of polysilicon allows the actuator to operate at voltages and currents compatible with standard integrated circuitry (e.g., CMOS). In addition, actuators according to the present invention are very small in area, have relatively high force, and can provide relatively long actuation displacements (e.g. 10-12 microns) at very small increments, making them suitable for deployment of micro-optical devices as well as providing minute adjustments. In one implementation, the present actuator array can produce a force of about 3700 xcexcN per square mm and with 1.53 mW per xcexcN of power. This electrically stimulated movement can be used in micro-motors, optical scanning devices, MEMS optical deployment mechanisms and other areas requiring mechanical movement on a micro scale.